Current therapies for breast cancer (BC) are firmly believed to improve patient survival efficiently. However, one-third of aggressive triple-negative breast cancer (TNBC) patients may relapse more frequently compared to hormone receptor-positive subtypes, and patients may eventually develop the distant metastatic disease and drug resistance. There are no targeted therapies currently available for invasive and metastatic diseases, indicating a dire need for improved therapy. We revealed that CCN5 acts as a tumor suppressor protein in TNBC and possibly by targeting p27Kip1, which is a cell growth regulator, and microRNA-10b (miR-10b) which is known as a mediator of invasion, metastasis, and drug resistance. However, therapeutic application of CCN5 remains unknown; this critical knowledge is required before CCN5-based drugs can be developed and moved into a clinical setting. Thus, the goal of this application is to create and establish an innovative approach for CCN5 therapy for TNBC growth and metastasis by synthetic modification of CCN5 protein through the amalgamation of polyethylene glycol (CCN5 PEGylation; PEG-CCN5). In our preliminary studies, we have demonstrated the feasibility of the proposed method. We have shown that that PEGylation decreases plasma clearance of CCN5 protein. Treatment of TNBC cells with p27Kip1 and miR-10b targeting PEG-CCN5 results in blocking cell growth, invasion, and sphere-forming ability, restoring ER-? expression and sensitizing it to chemotherapy. The results of the in vivo studies suggest that subcutaneous delivery of PEG-CCN5 to tumor- bearing mice leads to robust tumor uptake and suppresses tumor growth and metastasis. These preliminary findings form the basis for the current application, in which we propose to extend these studies to identify an optimal PEG-CCN5 derivative by determining functional efficacy and mechanism of action (Aim 1), MTD, biodistribution, and toxicity in vitro and in vivo (Aim 2). Finally, antitumor and anti-metastatic efficacy in combination treatment of anti-hormone, chemotherapy or both will also be determined in cell-derived orthotopic TNBC xenograft, syngeneic models and PDX model (Aim 3). These studies are expected to identify an optimal PEGylated protein that will inhibit TNBC growth and progression as well as sensitize TNBC cells to tamoxifen and Paclitaxel with minimal systemic toxicity or ill effects on healthy cells or organs. These approaches should significantly advance knowledge on therapeutic utility of CCN5 and its mechanistic insights in the suppression of TNBC. .